Apparatus for generating picture signals



Oct. 8, 1963 B. J. MAYO APPARATUS FOR GENERATING PICTURE SIGNALS Filed Jan. 13, 1961 FIG. 1.

WIIIII'IIIIIA United States Patent Oil-ice 3,105,605 Patented Oct. 8, 1963 3,106,605 APPARATUS FOR GENERATENG PICTURE SIGNALS Bernard Joseph Mayo, Beaconsfield, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Filed Jan. 13, 1961, Ser. No. 82,507 (Ii-aims priority, application Great Britain Jan. 15, 1960 11 Claims. (Cl. 1785.4)

This invention relates to apparatus for generating picture signals, that is electrical signals corresponding to light images, such as may be employed for television transmission purposes.

Many proposals have been made for constructing apparatus for generating an electrical signal corresponding to a light image which will be effective even at low light levels. A wide range of applications exist for such apparatus not only in the television broadcasting field but in scientific and industrial fields. For example, such apparatus is especially needed for colour television because the generation of colour television signals usually involves the division of the available light among a plurality of pick-up tubes. However, in apparatus such as proposed hitherto, the lower limit of the light level at which the apparatus can be used effectively is set at an undesirably high value by the pressure of noise, especially in signal elements corresponding to black elements of the light image, and by the presence of spurious signal components due to for example redistributed electrons and dynode shading.

The object of the present invention is to provide an improved apparatus for generating an electrical signal corresponding to a light image.

According to the present invention there is provided apparatus for generating an electrical signal correspon ing to a light image comprising a charge storage target, means for setting up a charge pattern on said target corresponding to a light image, means for scanning said target to produce a flow of electrons from the target which varies in time in dependence upon charges stored in successive elemental areas of the target, means including a luminescent screen for converting said electron flow into luminescence of said screen, and photo-electrically sensitive means responsive to said luminescence for producing an electrical signal which varies to represent said image.

The invention is especially applicable to apparatus in which the means for setting up a charge pattern on said target is arranged to produce a negative charge pattern, in which said means for scanning the target comprises means capable of releasing electrons from elemental areas of the target successively, and an electron-pervious electrode disposed and biassed in relation to the target to control the release of electrons so that the number of electrons which flow from the target varies in dependence upon the potential of the respective elemental area of the target relative to said pervio-us electrode.

In order that the said invention may be clearly understood and readily carried into effect, it will now be more fully described with reference to the accompanying drawings, in which:

FIGURE 1 diagrammatically illustrates one embodiment of the invention, and

FIGURE 2 illustrates a modification of the apparatus illustrated in FIGURE 1.

As shown in FIGURE 1 the apparatus comprises an electron discharge device the reference numeral 1 indicating the evacuated envelope of the device which has a window 2. at one end thereof having on its interior surface a photo-emissive cathode 3. Spaced from the oathode 3 is a charge storage target comprising a sheet 4' of partially conducting glass having on its side remote from the cathode 3 a mosaic 5 of photo-emissive elements. Spaced from but close to the mosaic 5 is a mesh electrode 6 and at the end of the envelope remote from the oathode 3 there is provided a luminescent screen 7 preferably of the aluminised type, that is coated on the side facing the target mosaic 5'- with a thin film of aluminium. Externally of the device is a photo-electron multiplier 8 of any suitable construction having its photo-emissive cathode 9 disposed facing the luminescent screen 7, there being provided if desired an optical system between the screen 7 and the cathode 9. The photo-electron multiplier 8 may comprise any suitable number of multiplying stages and the final electron collector electrode 10 may be connected to a signal resistance 11 across which picture signals are developed as hereinafter referred to.

Surrounding the space between the cathode 3 and the target electrode is a solenoid coil 12 for the purpose of maintaining electrons released from the cathode 3 in focus whilst being projected onto the glass sheet 4. An optical image from which picture signals are required to be developed can be projected onto the cathode 3 by means of a lens system conventionally indicated at 13-. Between the cathode 3 and the target are accelerating and decelerating electrodes 14, 15, 16, 17 and 18 the electrode 16 being connected to a mesh electrode 19 or the latter may be replaced by 'a large-apertured diaphragm. A wall anode 20 may be also provided between the mesh electrode 6 and the luminescent screen 7. Alternatively the wall anode may be applied on the exterior surface of the envelope 1, or may even be omitted in some cases. When an optical image is projected onto the cathode 3 photo-electrons are released and are projected onto the glass sheet 4 so as to charge such sheet negatively. Since the sheet 4 is partially conducting the charges leak through the sheet and similarly charge the photo-emissive elements 5. The high electrostatic capacitance between the two faces of the target also plays a part in the charge transfer. In operation of the arrangement the electrode 14 may be maintained at a potential of volts, the electrode 15 at 300 volts, the electrode 16 at 500 volts, the electrode 17 at 300 volts and the electrode 18 at 100 volts all of these voltages being positive relatively to the cathode 3. In order to restore the target to an equilibrium potential the photo-emissive elements are arranged to be scanned by a light beam. For the purpose the envelope 1 is provided with a window 21 disposed obliquely to the axis of the envelope 1, and externally of the window is a cathode ray tube 22 having a luminescent screen arranged to be scanned by an electron beam so as to generate a light spot which is projected onto the photo-emissive elements 5 by an optical system indicated at 23. The raster which is scanned on the fluorescent screen of the tube 22 may be distorted so as tocause the raster which is produced on the target to have a rectangular outline, notwithstanding the obliquity of the paths of projection. The mesh elect-rode 6 is maintained at a few volts positive with respect to the cathode 3. When the photo-emissive elements 5 are scanned by the light beam the elements in turn tend to emit photoelectrons. The number of photo-electrons from each scanned element which flow away from the target towards the fluorescent screen during such scanning operation depends on the potentials acquired by the photo emissive elements as the result of the projection of the photo-electrons onto the glass sheet 4 and is such that each element tends to be restored to an equilibrium potential which is substantially the potential of the mesh 6. Therefore there is derived from the photo-emissive mosaic a photo-electron beam, the intensity of which is modulated in time according to the charges acquired by the successive photo-emissive elements 5. The electron beam is projected onto the luminescent screen 7 which may be maintained at a positive potential of about 10 kv. which is thus caused to luminesce in intensity depending on the number of electrons impinging on the screen at any instant. The wall anode 20 is maintained at a potential which is lower than that of the screen 7, and may be about volts. The light so generated is projectcd onto the photo-emissive cathode 9 of the multiplier 8 and from this cathode a stream of electrons is generated which is caused to impinge on the first stage of the multiplier giving rise to secondary electrons which are in turn caused to be projected to the second stage of the multiplier and so on until the finally multiplied stream is collected on the electrode 10 where it serves to generate picture signals across the signal resistance 11. The operation of a photo-electron multiplier such as 8 is well known in the art. The photo-electrons emitted by the photo-emissive mosaic during scanning may be projected onto the luminescent screen 7 with a velocity of as aforesaid about kv. with the result that the number of electrons released from the photo-cathode 9 of the multiplier 8 for each electron leaving the photo-emissive mosaic may be of the order of 15. The area of the luminescent screen 7 is preferably larger than the area of the photo-emissive mosaic 5 so that all of the electrons leaving the target area can be readily collected on the luminescent screen.

The decay time of the phosphor comprising the luminescent screen 7 should be less than the time it takes to scan a picture point, i.e., 0.25 scc. for present day television standards in order to avoid the introduction of undue streaking" in a moving picture. If the decay time is not sufficiently short a correction could be applied if the decay time were substantially independent of brightness. Such correction is effected by increasing the amplitude of the high frequency components of the output signal relative to the amplitude of the low frequency components. The phosphor employed for the screen 7 may be gehlenite, ackermanite or zinc oxide. The ma tallic coating on the side of the screen 7 facing the mosaic elements 5 is adequate to restrict the feedback of light from the screen 7 to the mosaic elements 5 or other parts of the tube which are photo-emissive.

According to FIGURE 2, the apparatus of FIGURE 1 is modified by forming the envelope 1 with a re-entrant part 30 which acts as a nest to enclose the photo-electron multiplier 8. The luminescent screen 7 in this case is provided on the inner surface of the base 31 of the reentrant part, the aluminium coating of the luminescent screen being electrically connected to the terminal pin 32 by metallic paint 33. The cylindrical wall of the reentrant part is painted black and the black paint may be continued round to the outer surface of the envelope 1 as far as the optical window 21. The cylindrical wall of the multiplier 8 is preferably also painted black. The modification illustrated in FIGURE 2 has the advantage of enabling the overall length of the apparatus to be reduced without reducing the overall length of the envelope which contains the target 4 and the fluorescent screen. A relatively long envelope 1 is desirable in order that the heat sealing operation, which has to be performed on the envelope 1, may take place at a position which is a substantial distance both from the target 4 and from the photo-emissive cathode 3.

According to a further modification of FIGURE 1, which is not illustrated, the window 21 through which the scanning raster is projected from the tube 22 onto the target 4 may be formed in the same end wall of the envelope 1 as that on which the luminescent screen 7 is deposited, either the window 21 or the screen 7 or both the window and the screen being ofi axis.

The mode of operation of the apparatus is such that, when the target 4 is scanned, the number of electrons which flow away from an element of the target corresponding to a black element of the light image, is zero or at most very small. Therefore, noise in elements of the output signal of the apparatus corresponding to black elements of the light image is a minimum. As a result of the invention moreover there is little tendency for the production of spurious signal components due for example to redistribution of electrons at the target and dynode shading in the electron multiplier and noise in signal elements other than those corresponding to black is also maintained at a low value. It has moreover been found, that compared with an arrangement in which the electrons flowing from the target 4 as a consequence of scanning are caused to pass directly into an electron multiplier, and in which a head amplifier for the ouput signal was required, apparatus of the construction illustrated in FIGURE 1 was found not to require a head amplifier, even though the same number of stages of electron multiplication was used in both cases.

Because of its low noise output apparatus according to the invention is especially suitable for use in a simultaneous type colour television camera since it is effective at relatively low light levels. Moreover a single tube, corresponding to 22, can be used to produce the scanning raster for the different pick-up tubes which are responsive to the colour components of the light image, and this has the additional advantage of reducing registration problems.

Although the invention has been described above as applied to a device embodying a target of the light scanned type which is known in the art, it will however be appreciated that other forms of targets can be employed in which the electrons flow from the target during scanning in dependence upon the charges stored on the target.

What I claim is:

1. Apparatus for generating an electrical signal corresponding to a light image comprising a charge storage target, means for setting up a charge pattern on said target corresponding to a light image, means for scanning said target to produce a flow of electrons from the target which varies in time in dependence upon charges stored in successive elemental areas of the target, means includ ing a luminescent screen for causing said electrons to impinge on said luminescent screen and thereby to convert said electron fiow into luminescence of said screen, and photo-electrically sensitive means responsive to said luminescence for producing an electrical signal which varies to represent said image.

2. Apparatus according to claim 1 wherein said means for setting up a charge pattem on the target is arranged to produce a negative charge pattern, and said means for scanning the target comprises means capable of releasing electrons from elemental areas of the target successively, and an electron-pervious electrode disposed and biassed in relation to the target to control the release of electrons so that the number which flow from the target varies in dependence upon the potential of the respective elemental area of the target relative to said pervious electrode.

3. Apparatus according to claim 2 wherein said means for setting up a charge pattern on the target comprises a photo-emissive cathode for converting a light image into an electron image and means for projecting said electron image with such a low velocity onto said target as to avoid substantial release of secondary electrons, said last-mentioned means including a focussing solenoid for maintaining the electron image in focus at the target.

4. Apparatus according to claim 3 wherein said photoemissive cathode, said target and said luminescent screen are arranged in line and said luminescent screen is coated on the side facing the target with a thin metallic layer which substantially prevents light feedback from the inminescent screen to the target.

5. Apparatus according to claim 1 wheerin said target comprises photo-emissive material, and said means for scanning the target comprises means for causing a light spot to trace a scanning raster on said target so as to be capable of releasing electrons from elemental areas of the target successively.

6. Apparatus according to claim 1 wherein said photoelectrically sensitive means comprises a photo-electron multiplier.

7. Apparatus according to claim 1 wherein said target and said luminescent screen are enclosed in one envelope and said photo-electrically sensitive means is enclosed in another envelope.

8. Apparatus according to claim 7 wherein said other envelope nests in a re-entrant part of said first-mentioned envelope, said luminescent screen being provided on the inner surface of the base of said re-entrant part.

9. Apparatus according to claim 1 wherein the area of said luminescent screen is larger than the scanned area of said target.

10. A colour television camera comprising a plurality of apparatuses, each according to claim 1 and arranged so as to be responsive respectively to different colour components of a light image, and wherein said means for scanning the target in the respective apparatuses comprises a cathode ray tube common to the apparatuses for producing a scanning light spot and means for projecting images of said spot on the targets of the respective apparatuses.

11. Apparatus according to claim 3 wherein said target comprises photo-emissive material, and said means for scanning the target comprises means for causing a light spot to trace a scanning raster on said target so as to be capable of releasing electrons from elemental areas of the target successively.

References Cited in the file of this patent UNITED STATES PATENTS 2,618,761 Rose Nov. 18, 1952 2,739,258 Sheldon Mar. 20, 1956 2,755,408 'Iheile July 17, 1956 2,826,632 Weirner Mar. 11, 1958 2,929,866 Me-lamed Mar. 22, 1960 

1. APPARATUS FOR GENERATING AN ELECTRICAL SIGNAL CORRESPONDING TO A LIGHT IMAGE COMPRISING A CHANGE STORAGE TARGET, MEANS FOR SETTING UP A CHARGE PATTERN ON SAID TARGET CORRESPONDING TO A LIGHT IMAGE, MEANS FOR SCANNING SAID TARGET TO PRODUCE A FLOW OF ELECTRONS FROM THE TARGET WHICH VARIES IN TIME IN DEPENDENCE UPON CHARGES STORED IN SUCCESSIVE ELEMENTAL AREAS OF THE TARGET, MEANS INCLUDING A LUMINESCENT SCREEN FOR CAUSING SAID ELECTRONS TO IMPINGE ON SAID LUMINESCENT SCREEN AND THEREBY TO CONVERT SAID ELECTRON FLOW INTO LUMINESCENCE OF SAID SCREEN, AND PHOTO-ELECTRICALLY SENSITIVE MEANS RESPONSIVE TO SAID LUMINESCENCE FOR PRODUCING AN ELECTRICAL SIGNAL WHICH VARIES TO REPRESENT SAIG IMAGE. 